Network connection library for TCP and SSL

The conduit library takes care of establishing and listening for TCP and
SSL/TLS connections for the Lwt and Async libraries.

The reason this library exists is to provide a degree of abstraction
from the precise SSL library used, since there are a variety of ways to bind to
a library (e.g. the C FFI, or the Ctypes library), as well as well as which
library is used (either OpenSSL or a native OCaml TLS implementation).